1. Technical Field of the Invention
The present invention relates to a code division multiple access (CDMA) receiving apparatus and method.
2. Description of the Prior Art
Recently, spread spectrum communication system highly insensitive to interference and disturbance has attracted public attention as one of communication methods applicable to mobile communication system. In radio communication systems using the spread spectrum signals, voice data or video data is modulated by PSK (Phase Shift Keying) or FSK (Frequency Shift Keying) or the like, and the modulated data is converted to wide-band base band signal using pseudo noise code (PN code: pseudo random noise code).
On the other hand, a receiving unit carries out despreading the received radio frequency signal using the same code as the PN code used in the transmission unit and reproduces receiving data by carrying out digital demodulation according to PSK or FSK demodulation method.
This kind of system employs RAKE receiving system as one of countermeasures against multi-path interference. That is, in radio communication system, radio wave transmitted from the transmission unit may reach the receiving unit directly or after reflected by a building or mountain.
If a radio wave reaches the receiving unit through a plurality of paths, the radio waves passing through the aforementioned a plurality of paths are synthesized in terms of vector at an antenna end of the receiving unit so as to reduce reception level. This phenomenon is called multi-path. If the multi-path interference occurs, in a system whose radio channel bandwidth is as small as 30 kHz, for example, its receiving unit is not capable of receiving radio signal. However, because in the spread spectrum communication system, its radio channel bandwidth is wide, the radio signal is always received although part of them is lost.
Thus, in the spread spectrum communication system, the multi-path receiving signal received through a single antenna is separated by the unit of each PN code length (1 chip) and inputted to a plurality of dependent demodulators. These demodulators carry out despreading with the PN code corresponding to each path so as to demodulate the receiving signals and the demodulated receiving signals through a plurality of paths are synthesized in terms of symbol so as to reproduce receiving data.
This is called RAKE receiving method because the reception signals are collected and synthesized as if the rake is employed. If the RAKE receiving method is used, time diversity is carried out, so that the reception quality in case of generating the multi-path can be increased greatly.
FIG. 12 shows a conventional CDMA receiving apparatus. Referring to FIG. 12, the CDMA receiving apparatus comprises a receiving processing control unit 85, a searcher unit 81, a finger unit 82, a RAKE combining unit 83 and a decoding unit 84. The receiving processing control unit 85 is connected to a call processing control unit 86 so that a transmission control signal from the RAKE combining unit 83 is outputted to a CDMA transmission unit 87.
A CDMA signal is inputted to the searcher unit 81 and the finger unit 82. The searcher unit 81 detects a reception timing of the CDMA signal having a high correlation value with respect to the PN code instructed by the receiving processing control unit 85 and outputs reception delay amount information of the CDMA signal to the finger unit 82.
The finger unit 82 carries out despreading, fading vector generation, detection and the like for the CDMA signal. The RAKE combining unit 83 outputs a signal obtained by combining detection signals of respective paths, inputted from the finger unit 82 to the decoding unit 84. The RAKE combining unit 83 outputs transmission control signal for power control, diversity control and the like to the CDMA transmission unit 87. The decoding unit 84 decodes a signal obtained by RAKE combining inputted from the RAKE combining unit 83 so as to obtain a desired signal.
However, because the conventional CDMA receiving apparatus has such a structure which satisfies a maximum rate of high-speed data determined depending on system condition and a maximum number of simultaneous voice data processing users (the maximum number of simultaneous processing users generally requested in the form of high-speed data is smaller than the case of voice data) with a single reception processing block, there is such a problem that the availability of the resource of the CDMA receiving apparatus drops.
For example, in case where reception processing for high-speed data is carried out, a load on the finger unit and the RAKE combining unit is increased. This means that if the operation speed of each unit is not high enough, more finger blocks and RAKE combining units are required. Conversely, because the searcher unit processes a pilot signal having the same rate in each service, it has no dependency on the rate and at the same time, depends on the number of users, which are treated simultaneously. Therefore, the resources of the finger unit and RAKE combining unit become short, so that relatively, the resource of the searcher unit becomes excessive.
In case where reception processing of voice data is carried out, a load on the searcher unit is increased. This is because a number of users is larger than that in the high-speed data. Thus, the resource of the searcher unit becomes short, so that relatively, the resources of the finger unit and the RAKE combining unit become excessive.
Further, The CDMA receiving apparatus executes demodulation processing immediately after any CDMA signal is received, irrespective of the service types. Therefore, voice reception processing in which real time processing is required cannot be carried out or may be delayed, when the number of users of high-speed data is increased and the resources in the finger unit and RAKE combining unit are consumed by the demodulation of the high speed data.